Reference is made to co-pending U.S. patent application Ser.No. 09/950,544, filed on Sep. 11, 2001, titled: xe2x80x9cMethod For Stitching Partial Radiation Images To Reconstruct A Full Imagexe2x80x9d inventors Wang et al
This invention relates in general to digital radiography, and in particular to the imaging of an elongated object such as a long human body part, such as the spine or legs, using a storage phosphor-based computed radiography system.
When an elongated object such as a long segment of the human body (legs, spine) is imaged radiographically using the conventional screen-film technique, special cassettes and films of extended length are used, such as 30xc3x9790 cm and 35xc3x97105 cm. The radiographic assessment of scoliosis patients begins with erect anteroposteries and lateral views of the entire spine. This can be accomplished with a radiograph that is (91.5 cm) in length. Long bone images are necessary when an evaluation of a larger portion of the skeletal system must be viewed as a whole and radiographs of up to (130 cm) in length are desirable. As medical institutions are migrating from analog screen-film systems to digital modalities, such as computed radiography (CR), these types of exams impose a significant challenge. This is because the size of digital detector is limited. For example, the largest CR storage phosphor cassette from several major CR vendors is limited to 35xc3x9743 cm, which can only image a portion of the long body part at a time. To address this problem, several methods have been proposed. European Patent EP0919856A1 discloses a way of staggering several storage phosphor cassettes together. The cassettes can be in an alternating, staircase-wise, or oblique arrangement. During the x-ray exposure, all the partially overlapping cassettes are exposed simultaneously, therefore each storage phosphor screen that resides inside the corresponding cassette records a part of the image of the long body part. The drawback of this approach is that the metallic frames of the front (closer to the x-ray source) cassettes impose shadows in the image recorded in the back cassettes. The shadows are not removable and therefore may hinder diagnostic interpretation of the acquired images. European Patent EP0866342A1 (also US Pat. No. 5,986,279, issued Nov. 16, 1999, inventor Dewaele) presents a method that is based on partially overlapping a plurality of storage phosphor screens for extended imaging coverage. The screens can also be configured in an alternating, staircase-wise, or oblique overlapping arrangement. Further, the screens can be contained in a single, extended length cassette for convenience of use. This approach overcomes the drawback of the cassette stacking method because there are no cassette metallic frames present in the x-ray path. However, in practice, this method requires that the storage phosphor screens be removed from the cassettes before imaging, and to be placed back into the cassettes in a darkroom after the x-ray exposure, which is cumbersome in the clinical environment.
The sub-images acquired by the individual storage phosphor screens must be stitched together to create a composite full image. The stitched full image should be distortion-free for the purposes of diagnostic interpretation and geometric measurement. U.S. Pat. No. 4,613,983, issued Sep. 23, 1986, inventors Yedid et al., discloses a method to reconstruct a composite radiographic image from a set of sub-images. However, this method is applicable only when the relative position between the sub-images is precisely controlled by the acquisition hardware. European Patent EP0919858A1 proposes a method that utilizes a pattern of reference markers that impose shadows simultaneously with the diagnostic image in each of the acquired sub-images. The drawback of this method is that a precisely fabricated pattern of reference markers must be imaged simultaneously with the patient in order to achieve precise geometric registration of the sub-images. The shadow of the reference markers may obscure diagnostically important information in the stitched image.
It is therefore desirable to develop a computed radiography system for elongated images such as of the spine and leg.
According to the present invention, there is provided a solution to the problems discussed above.
According to a feature of the present invention, there is provided an apparatus for radiographically imaging elongated objects comprising: a vertical main frame; a base mounted on said main frame; a first support on said base for supporting a first cassette containing a first storage phosphor imaging plate; a door hingedly mounted on said base for movement between open and closed positions; said door having a light-tight enclosure for supporting a second cassette containing a second storage phosphor imaging plate removably mounted within a cassette shell; and a first mechanism for removing said cassette shell from said second storage phosphor imaging plate which remains supported within said light-tight enclosure; such that, when said door is in said closed position, said second storage phosphor imaging plate overlaps said first storage phosphor imaging plate, whereby an elongated object can be radigraphically imaged onto said overlapping first and second storage phosphor imaging plates.
According to another feature of the present invention there is provided a computed radiography cassette comprising: a cassette shell; a storage phosphor imaging plate removably contained within said cassette shell; and a handle detachably mounted on said cassette shell for facilitating removal of said cassette shell relative to said storage phosphor imaging plate.
According to still another feature of the present invention there is provided a method of radiographically imaging an elongated object comprising: positioning a first cassette containing a first storage phosphor imaging plate in overlapping relationship with a second storage phosphor imaging plate which has been removed from its cassette plate; and exposing said overlapped first and second storage phosphor imaging plates to a radiographic image of an elongated object wherein said second plate is closer to said source of said radiographic image than said first plate.
The invention has the following advantages.
1. Radiographic images of elongated objects, such as the leg and spine, can be taken in a simple, efficient and cost effective manner.
2. Unwanted artifacts in the image are minimized due to the external structure of cassettes when overlapping cassettes are used.
3. Use of an alignment grid or other external marks is eliminated.